Investigations of the catalytic mechanism of several multielectron reductase enzymes, including the NADPH-sulfite reductase of Escherichia coli and the ferredoxin-nitrite reductase of spinach, will be conducted utilizing both static and rapid kinetic approaches with analysis of samples by EPR absorption spectroscopy. NMR studies will be conducted on siroheme and its derivatives in an effort to elucidate its isomeric structure. A possible role for siroheme in animal systems will be explored. BIBLIOGRAPHIC REFERENCES: L.M. Siegel, P.S. Davis, and M.J. Murphy. (1977) Biochem. J., in press. Incorporation of methionine-derived methyl groups into sirohaem by Escherichia coli. L.M. Siegel, M.J. Murphy, and H. Kamin. (1977) Methods Enzymol. (Biomembranes, Part C), S. Fleischer and L. Packer, eds., Academic Press, N.Y., in press. Siroheme: Methods of isolation and characterization.